Image Tile Caching, rehashed

A little more than a year ago, I wrote about my rather simplistic approach to caching image tiles for the interactive map viewer on PhilaGeoHistory.org. Since that time, I’ve made a few updates that are worthy of a reposting, particularly if you want to use the code for anything.

First, one of our partners, the Free Library of Philadelphia, wanted to use the script, but wanted it in ColfFusion. So I migrated it from PHP to ColdFusion. The two versions work very similarly, so you can see the current version in PHP or ColdFusion.

Second, here are the new features:

Store the cached images for different layers in different directories. Seems simple enough. I hesitated to do this previously because you could request multiple layers at once through the WMS script, but in practice I’ve never done it.

In ColdFusion, open the resulting image to make sure it is a valid image. Sometimes the file that is returned is garbage, or just an error, and you don’t want to cache it. Also, if the PNG file is smaller than 1K, it’s most likely not an image at all.

If the first attempt to retrieve an original tile fails, or results in an error, retry the http request again.

Use an appropriate file extension denoting the format of the image. Previously, just the hash of the request string was used as the filename. Now it’s the has plus the correct extension. Makes it possible to set the correct MIME type when serving up a cached image, and makes viewing problematic images in the file system a lot easier.

Return an HTTP header telling the client to cache the image. The results of dynamic scripts in general are not cached by browsers, but in this case, significant speed benefits arise from holding onto the images, which are very unlikely to change in any short period of time.

Further features that would be useful include recognizing the geographic extent of each layer and not caching tiles outside of that extend; recognizing empty tiles; and more efficiently storing the resulting cached files.

I don’t expect to invest too much in this, especially since there are much larger systems to do things like this (TileCache is one example). Currently, it’s too complicated for me to consider implementing, but if I need more features, or the project scales larger, I’ll have to migrate to something like it.

This entry was posted on Friday, October 2nd, 2009 at 12:49 pm and is filed under Technology. You can follow any responses to this entry through the RSS 2.0 feed.
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